The present invention relates to a direct current detecting device using a plurality of saturable reactors, and, more particularly, to a device for measuring a direct current flow of a bus or a conductor with a high precision by utilizing saturable reactors.
The device of the present invention is suitable for use in the detection and measurement of an exciting current of a superconductive magnet or a nuclear particle accelerator. In these applications, it is desirable to detect or measure the d.c. current flow of a conductor with high precision while maintaining electrical insulation between the detector and the conductor.
Proceeding of IV International Symposium on Magnet Technology, September, 1972, page 865 to page 868 describes the so called Kraemer type d.c. current transformer utilizing a pair of saturable reactors which is one type of conventional d.c current detecting devices. The Kraemer d.c current transformer, however, suffers from a relatively low measurement precision for several reasons. One of the reasons is that an output of the Kraemer d.c current transformer includes a high ripple voltage level because slits or notches are inevitably present in its waveform at each cycle. Another is due to an unbalance of characteristics of exciting current versus exciting voltage of the saturable reactor pair.
In order to eliminate the latter problem, the use of two saturable reactor pairs has been proposed as described in U.S. Pat. No. 3,801,907 as well as in Electronic Design, June 7, 1973. This patent discloses a device for the measurement of the direct current, in which a pair of Kraemer d.c. current transformers are utilized to produce a differential output which is supplied to a feedback winding on each reactor. The device, however, still suffers from a high ripple voltage level due to the waveform slits or notches. In other words, the device has difficulty in eliminating the former problem mentioned above. It is, of course, possible to reduce the undesirable influence of the ripple by lowering the gain and frequency response of a differential amplifier. However, this results in also lowering the precision of the measurement of the current flow.
Another way of reducing the ripple level of the output voltage waveform is to utilize another saturable reactor connected in series with the output circuit of the differential amplifier as suggested in the above U.S. Pat. No. 3,801,907. However, the addition of the saturable reactor for suppressing the ripple voltage level causes a lowering of the frequency response with respect to the current flow changes to be measured. In consequence, it seems that according to the conventional device, there is no effective way of suppressing the slits or notches of the output waveform without lowering both the measurement precision and the frequency response characteristic.
On the other hand, the aforementioned paper, Proceeding of the IV International Symposium on Magnet Technology, September 1972 PP865-868 describes an improved d.c. current transformer capable of eliminating the slits or notches of the output waveform. This device, however, has a disadvantage in that an undesirable exciting current of the magnetic core is superposed on the output current so that the measurement precision is lowered.